Narcan is an emergency antidote that rapidly reverses the effects of an opioid overdose. It's so effective that it can actually bring people back from the brink of death.

Kailin See, the director at the Washington Heights Corner Project — a New York-based organization that offers free Narcan kits and training — explained what it is.

"Narcan is a very simple and very safe drug,” she said. “It is very similar to an EpiPen, it presses pause on the opioid overdose and allows the person to keep breathing."

Narcan can also be administered through a nasal spray.

The drug can be purchased in just about any pharmacy across the country. Most states permit the drug to be sold over the counter, so no prescription is necessary and you don't need to be a medical professional to use it.

But Dr. Kevin Gilliland, an addiction specialist, said, "You can only use Narcan in an emergency."

"If you're a loved one or a friend of someone who's struggling with opiates, pain meds, or heroin or if they've been clean for a while, you should have it just in case something happens,” he said.

Lovato’s friends reportedly "had Narcan on hand in case something like this happened,” according to a source, adding, "It saved her life."

Narcan has a 93 percent success rate in reviving people who have overdosed, according to EMS data.